media: bttv: move do_crop flag out of bttv_fh

The do_crop flag indicates whether a cropping rectangle has been set.
Instead of storing this flag separately in each file handle, move do_crop
to struct bttv in preparation for vb2 conversion which stops using
separate bttv file handles.

Signed-off-by: Deborah Brouwer <deborah.brouwer@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>